Closed-loop power controlled radio communication system

ABSTRACT

In a radio communication system ( 50 ) employing closed loop power control a primary station ( 100 ) adjusts its transmit power in response to control information including power control commands received from a secondary station ( 100 ). If the primary station ( 100 ) fails to obey the power control commands the secondary station modifies the control information and the primary station ( 100 ) uses the modified control information to adapt a characteristic of a transmission.

The present invention relates to a radio communication system, to aprimary station and to a secondary station for use in a communicationsystem, and to a method of operating a communication system.

In a radio communication system comprising base stations of fixedlocation and mobile stations, such as UMTS (Universal MobileTelecommunication System), transmissions from a base station to a mobilestation take place on a downlink channel and transmissions from a mobilestation to a base station take place on an uplink channel. It is knownfor such systems to use closed loop transmit power control in which amobile station measures the quality of a received downlink signal andtransmits transmit power control (TPC) commands to the base station sothat an adequate, but not excessive, received signal level is maintainedat the mobile station despite fluctuations in downlink channelattenuation. The TPC commands typically comprise “increase power” and“decrease power” commands where the step size of the increase ordecrease is predetermined.

Also, it is known to vary the step size of the TPC commands in responseto fluctuations in the prevailing channel attenuation. For example usinga large step when the channel attenuation is changing quickly, and usinga small step when the channel attenuation is changing slowly. In thisway the transmit power can better track changes in channel conditions.

Furthermore, it is known for radio communication systems to usetransmission format control in which a mobile station measures thequality of a received downlink signal and transmits reports of thequality (typically referred to as Channel Quality Information reports orChannel Quality Indicator (CQI) reports) to a base station, and the basestation then schedules packet transmissions to certain mobile stationsand selects a transmission format, for example modulation and codingscheme, suitable for optimising communication under the prevailingchannel conditions. Such channel quality reports may provide anindication of, for example, carrier-to-interference ratio,signal-to-noise ratio, delay spread, or a recommended transmissionformat suited to the prevailing channel conditions. As an example, theHigh-Speed Downlink Packet Access (HSDPA) feature of UMTS uses adaptivemodulation and coding (AMC) to set suitable transmission parameters fortransmitted downlink (DL) data. The modulation and coding scheme for theDL data may be selected by the base station on the basis of the currenttransmit power used for a power-controlled channel, calibrated byintermittent channel quality reports received from the mobile station.An uplink may be operated in an equivalent manner.

Typically, the quality measurements used for generating the channelquality reports are made on a downlink signal whose transmit power isnot varied by a closed loop power control process, such as the CommonPilot Channel (CPICH) in UMTS, but the quality measurements used forgenerating the TPC commands have to be made on a downlink signal whosetransmit power is varied as a result of the TPC commands, in order toachieve closed loop operation.

The channel quality reports are generally transmitted at a lower ratethan the TPC commands and so the TPC commands may additionally be usedby the base station to assist scheduling of transmissions and selectionof a transmission format.

There can be circumstances under which a transmitting station is unableto obey power control commands, for example:

an “increase power” command may be received when the transmittingstation is already transmitting at its maximum power level, or at itsmaximum power level for the particular channel;

a “decrease power” command may be received when the transmitting stationis already transmitting at its minimum non-zero power level, or at itsminimum non-zero power level for the particular channel;

a processing overload may occur, preventing the transmitting stationfrom processing the power control commands.

If a transmitting station does not obey the power control commands, thesimple “increase power” or “decrease power” commands of the closed-looppower control, are no longer representative of the actual level of thechannel attenuation. In this circumstance, the transmitting stationmerely knows that the current channel attenuation is better or worsethan the current transmit power level would suggest. This prevents thetransmitter from using the transmit power level as a reliable indicatorwhen choosing suitable parameter values for transmitted signals. As aresult, transmission parameters may be non-optimal until thetransmitting station is able to resume obeying the power controlcommands and the power control loop has re-converged. This period ofnon-optimal operation can result in inefficiency.

An object of the present invention is to contribute to improvedefficiency.

According to a first aspect of the present invention there is provided asecondary station for use in a communication system comprising a primarystation and a secondary station, the secondary station comprising:receiver means for receiving a first signal transmitted by the primarystation; measurement means for measuring a first characteristic of thereceived first signal; processing means for deriving power controlcommands from the measured first characteristic; transmitter means fortransmitting control information comprising at least the power controlcommands to the primary station; and further comprising control meansresponsive to the primary station failing to adjust its transmit powerin compliance with the transmitted power control commands for modifyinga parameter of the control information transmitted to the primarystation.

According to a second aspect of the present invention there is provideda communication system comprising a secondary station in accordance withthe first aspect of the invention and a primary station, the primarystation comprising: transmitter means for transmitting a first signal;receiver means for receiving power control commands; control means foradjusting the transmit power of the first signal in compliance with thereceived power control commands provided that the adjustment is withinthe contemporaneous capability of the primary station; and wherein theprimary station control means is adapted to adapt a characteristic of atransmission in response to receiving the modified control informationtransmitted by the secondary station.

According to a third aspect of the present invention there is provided aprimary station for use in a communication system comprising a primarystation and a secondary station, the primary station comprising:transmitter means for transmitting a first signal; receiver means forreceiving power control commands; control means for adjusting thetransmit power of the first signal in compliance with the received powercontrol commands provided that the adjustment is within thecontemporaneous capability of the primary station; and wherein thecontrol means is adapted to generate an indication for transmission inresponse to failing to adjust the transmit power of the first signal incompliance with the received power control commands.

According to a fourth aspect of the present invention there is provideda method of operating a communication system comprising a primarystation and a secondary station, the method comprising:

transmitting a first signal from the primary station;

at the secondary station,

receiving the first signal;

while receiving the first signal, measuring a first characteristic ofthe received first signal, deriving power control commands from themeasured first characteristic, and transmitting control informationcomprising at least the power control commands;

at the primary station,

receiving the power control commands;

adjusting the transmit power of the first signal in compliance with thereceived power control commands provided that the adjustment is withinthe contemporaneous capability of the primary station;

further comprising,

at the secondary station, in response to the primary station failing toadjust the transmit power in compliance with the received power controlcommands, modifying a parameter of the control information transmittedto the primary station, and

at the primary station, in response to receiving the modified controlinformation, adapting a characteristic of a transmission.

The invention is based on the realisation that a primary station may notalways be able to obey power control commands, that this circumstancemay be detectable or predictable, and that action may be taken while theprimary station is not obeying power control commands to assist recoverywhen the primary station is able to resume obeying power controlcommands, thereby reducing inefficiency. In response to the primarystation not obeying power control commands, or at times when the primarystation may not be able to obey power control commands, the secondarystation modifies a parameter of the control information that ittransmits to the primary station, and in response to receiving themodified control information, the primary station adapts acharacteristic of its transmission.

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings wherein;

FIG. 1 is a block schematic diagram of a communication system comprisinga primary station and a secondary station;

FIG. 2 is a flow chart illustrating a method of operating acommunication system in accordance with the invention;

FIG. 3 is a graph illustrating the variation of signal-to-interferenceratio and transmit power with time; and

FIG. 4 is a graph illustrating a benefit of the invention.

Referring to FIG. 1, there is illustrated a radio communication system50 comprising a primary station 100 and a secondary station 200.

The primary station 100 comprises a transmitter means 110 fortransmitting a first signal, for example a data signal, to the secondarystation 200. The transmitter means 110 has an output coupled to anantenna means 120 via coupling means 130 which may be, for example, acirculator or changeover switch. The coupling means 130 also couplessignals received by the antenna means 120 to an input of a receivermeans 140. Coupled to the transmitter means 110 and the receiver means140 is a control means (μC) 150 for adjusting the transmit power of thefirst signal in response to TPC commands received from the secondarystation 200, provided that the adjustment is within the contemporaneouscapability of the primary station. The contemporaneous capability of theprimary station may depend on the amount of power being used fortransmitting other signals, on any maximum and minimum power limitswhich may apply, or on the current processing load.

Optionally the control means 150 may generate an indication fortransmission in response to the transmit power failing to be adjusted incompliance with the received power control commands.

Optionally the control means 150 may generate a second signal, such as apilot signal, for transmission to the secondary station, the transmitpower of this second signal not being controlled by a power controlloop.

Optionally the control means 150 may select a transmission format, forexample bit rate, modulation scheme, or coding scheme, for the firstsignal or any other signal in response to channel quality reportsreceived from the secondary station 200.

The secondary station 200 comprises a transmitter means 210 having anoutput coupled to an antenna means 220 via coupling means 230 which maybe, for example, a circulator or changeover switch. The coupling means230 also couples signals received by the antenna means 220 to an inputof a receiver means 240. Coupled to an output of the receiver means 240is a measurement means (M) 250 for measuring a first characteristic, forexample received signal power, signal-to-interference ratio (SIR) orsignal-to-noise ratio (SNR) or rate of change of these ratios, of thefirst signal received from the primary station. Coupled to an output ofthe measurement means 250 is a processing means (μP) 260, for derivingpower control commands from the measured first characteristic of thefirst signal. An output of the processing means 260 is coupled to aninput of the transmitter means 210 for transmitting control informationincluding at least the power control commands to the primary station.

Coupled to an output of the receiver means 240 is a control means (μC)270. In one embodiment, the control means 270, by monitoring thereceived signal, is adapted to determine that the primary station hasfailed to adjust its transmit power in compliance with the power controlcommands. In another embodiment the control means 270 is adapted toreceive from the primary station 100 an indication that the primarystation 100 has failed to adjust its transmit power in compliance withthe power control commands. In another embodiment the control means 270stores an indication of one or more predetermined times at which theprimary station 100 may fail to adjust its transmit power in compliancewith the power control commands.

In response to the primary station 100 failing, or potentially failingat one of the stored predetermined times, to adjust its transmit powerin compliance with the power control commands, the control means 270 isadapted to modify a parameter of the control information transmitted tothe primary station 100. The control means 270 may perform this functionin several alternative ways as described below, or in a combination ofthe ways. In FIG. 1, optional couplings dependent on the operation ofthe control means 270 are drawn using broken lines.

In one embodiment the control means 270 is adapted to modify a parameterof the control information transmitted to the primary station 100 bygenerating an indication of a power step size for transmission to theprimary station 100 to use when adjusting its transmit power; for thispurpose the control means 270 may be coupled to the transmitter means210. The indication of power step size may represent a specific amountor number of increments by which the primary station 100 should adjustits transmit power, or may indicate the discrepancy between thecurrently-received SIR at the secondary station 200 and a target SIRlevel. For example, the power control commands may be changed fromsimply “increase power” to “increase power by xdB” or “increase power byx increments”, where x is varied depending on the difference between thequality of the received first signal and a target quality, or the powercontrol commands may be changed to “increase power; discrepancy xdB”where x is the discrepancy between the currently-received SIR at thesecondary station 200 and a target SIR level. At the primary station100, the control means 150 may use the additional information “x” toadapt a characteristic of a transmission to improve efficiency. Forexample, the control means 150 may use the additional information topredict a suitable transmit power level to use when it is able to resumeobeying transmit power control commands.

As another example of the way in which a parameter of the controlinformation transmitted to the primary station 100 may be modified, thesecondary station 200 may group together a plurality of power controlcommands to enable each command to be transmitted indicating a powerstep size at a higher resolution using more bits, but at a lower rate.

In another embodiment, which may be used in conjunction with a primarystation 200 that transmits the second signal, the measurement means 250is adapted to measure a characteristic, such as SIR or SNR, of thesecond signal, and the processing means 260 is adapted to derive channelquality reports from the measured characteristic for transmission by thetransmitter means 210 at a predetermined rate. In this embodiment, thecontrol means 270 is adapted to modify a parameter of the controlinformation transmitted to the primary station 100 by causing thechannel quality reports to be transmitted at a rate higher than thepredetermined rate; for this purpose the control means 270 may becoupled to the processing means 260. At the primary station 100, thecontrol means 150 may use the additional information provided by theadditional channel quality reports to adapt a characteristic of atransmission to improve efficiency. For example, the control means 150may use the additional channel quality reports to predict a suitabletransmit power level to use when it is able to resume obeying transmitpower control commands, or to select a suitable modulation or codingscheme for the first signal or another signal.

In another embodiment, which may be used in conjunction with a primarystation 200 that transmits the second signal, the measurement means 250is adapted to measure a characteristic, such as SIR or SNR, of thesecond signal by averaging a function, such as a logarithmic function orsimply unity, of the characteristic over a predetermined time period,and the processing means 260 is adapted to derive channel qualityreports from the measured characteristic for transmission by thetransmitter means 210. The channel quality reports may comprise arecommend modulation and coding scheme. In this embodiment, the controlmeans 270 is adapted to modify a parameter of the control informationtransmitted to the primary station 100 by causing the averaging to beperformed over a time period shorter than the predetermined time period;for this purpose the control means 270 may be coupled to the measurementmeans 250. By reducing the averaging period, more detail about channelvariations is provided to the primary station 100. At the primarystation 100, the control means 150 may use the additional detail toadapt a characteristic of a transmission to improve efficiency. Forexample, the control means 150 may use the additional detail to predicta suitable transmit power level to use when it is able to resume obeyingtransmit power control commands, or to select a suitable modulation orcoding scheme for the first signal or another signal.

FIG. 2 is a flow chart illustrating a method of operating acommunication system in accordance with the invention. The method startsat step 500. Steps on the left hand side of FIG. 2 and labelled withreference numerals ending in “5” relate to steps performed at theprimary station 100, and steps on the right hand side of the Figure andlabelled with reference numerals ending in “0” relate to steps performedat the secondary station 200. Steps which are optional are drawn withboxes having broken lines.

At step 505 the primary station 100 commences transmitting a firstsignal, for example a data signal, to a secondary station 200. Thisfirst signal will be subject to closed loop power control. The primarystation 100 may optionally also commence transmission of a secondsignal, such as a pilot control signal, which will not be subject toclosed loop power control but is transmitted at a constant power level,or at a power level which varies in a predetermined manner known to thesecondary station 200.

At step 510 the secondary station 200 commences receiving the firstsignal, and the second signal if transmitted. At step 520 the secondarystation 200 measures a first characteristic of the first signal, such asreceived signal power, SIR or SNR, or difference between thesequantities and a target, or rate of change of these ratios. At step 530the secondary station 200 derives a power control command from themeasured a first characteristic of the first signal. At step 540 thesecondary station 200 transmits the power control command.

At optional step 550, if the second signal has been received, thesecondary station 200 measures a second characteristic of the secondsignal. The second characteristic may be, for example, received signalpower, SIR or SNR.

At optional step 560, if the second signal has been received, a channelquality report is derived from the measured second characteristic of thesecond signal, for example by averaging, and at step 570 the secondarystation 200 transmits the channel quality report.

Flow then proceeds to step 515 where the primary station 100 receivesthe power control command.

At step 525 the primary station determines whether it is currentlycapable of obeying the power control command. If it is, at block 535 itadjusts the transmit power of the first signal in accordance with thecommand. In either event, flow proceeds to optional step 545 where, if achannel quality report has been transmitted, the channel quality reportis received and, at block 555 the primary station 100 determines, basedon the channel quality report, a parameter for the first signal oranother signal. Such a parameter may be, for example, a data rate,modulation scheme or coding scheme.

Flow then proceeds to step 580 where the secondary station 200determines whether the primary station 100 has obeyed, or may not obey,the power control command. This condition need not be determined commandby command, but may be determined by taking an average over more thanone power control command so that, for example, failure to obey only oneof a sequence of power control commands is ignored. Example methods bywhich the secondary station 200 may determine this condition are:

a) Determining the SIR, or other characteristic, of the received firstsignal and detecting when the determined characteristic fulfils apredetermined criterion. As an example, the predetermined criterion maybe detecting when the SIR reduces below, (or increases above) a targetlevel for a predetermined period of time, or for a predeterminedproportion of a longer period of time. As another example, thepredetermined criterion may be detecting when the rate of change of SIRover a predetermined time period, or for a predetermined proportion of alonger period of time, is equal to, or within a predetermined toleranceof, the rate of change of SIR of a non-power controlled signal such asthe second signal.

b) Receiving an indication transmitted by the primary station 200 offailure to obey the power control command.

c) Referring to predetermined times stored by the secondary station 200at which the power control commands may not be obeyed.

If the secondary station 200 autonomously determines that power controlcommands are not being obeyed, or may not be obeyed, it may signal thiscondition to the primary station 100 to enable the primary station 100to assist the primary station 100 to detect the modified controlinformation transmitted by the secondary station 200.

If the power control command is not obeyed, or may not be obeyed, thesecondary station 200 at step 590 modifies a parameter of the controlinformation transmitted to the primary station 100. Examples of thismodification are:

a) Additional information such as a power step size may be included inthe control information, as described above.

b) The rate at which channel quality reports are transmitted may beincreased, as described above.

c) The way that channel quality reports are derived from the measuredsecond characteristic of the second signal may be modified. For example,the channel quality reports may be derived by averaging a function, suchas a logarithmic function or simply unity, of the measured secondcharacteristic over a predetermined time period, and in response to thepower control command not being obeyed, the averaging period may bedecreased. By decreasing the averaging period, more detailed informationcan be included in the channel quality reports because less informationis discarded through averaging. As an example, when the invention isapplied to UMTS, the secondary station 200 may normally report channelquality averaged over 40 sub-frames, where 40 sub-frames has a durationof 80 ms, but change to reporting channel quality averaged over 1sub-frame (2 ms) if the primary station 100 is often transmitting atmaximum power.

Whether or not a parameter of the control information transmitted to theprimary station 100 is modified in response to the primary station notobeying the power control command, flow returns to step 505 andcontinues in a loop until the signal transmission finishes. At thesecond or subsequent passes through step 505, the primary station 100may apply the modified control information received from the secondarystation 100 in adapting a characteristic of the transmitted firstsignal, or of another signal transmitted simultaneously ornon-simultaneously with the first signal. Examples of how the primarystation 100 may apply the modified control information received from thesecondary station 200 are:

a) A transmit power level for the first signal may be determined forwhen the primary station 100 is able to resume obeying power controlcommands.

b) If the transmit power of the primary station 100 is deployed across aplurality of simultaneously transmitted signals, when transmission ofone of the signals terminates, thereby making more power available forthe remaining signals, the primary station 100 may determine how todeploy the newly available power to the remaining signals.

c) If the transmit power level of the first signal is inadequate forreliable communication, the primary station 100 may decide to re-deploythat power to another signal which can achieve reliable communicationwith the additional power.

In each of these examples, the modification of a parameter of thecontrol information transmitted to the primary station enables theprimary station 100 to adapt its transmissions more rapidly, therebyimproving efficiency.

The time period for which the secondary station 200 transmits themodified control information, or the time period for which the primarystation 100 continues to adapt a characteristic of the transmittedsignal in response to receiving the modified control information, may bepredetermined, or may be dependent on the time period for which theprimary station fails to obey power control commands.

Referring to FIG. 3, graph A illustrates the variation with time of theSIR of the second signal, which is not power controlled; largevariations in SIR may be observed. Graph B illustrates the variationwith time of the SIR of the first signal, which is power controlled; dueto the power control, the extent of variation is decreased toapproximately ±1 dB for most of the time. Graph C illustrates therelative transmit power of the power controlled first signal; it can beseen that the upper limit to the transmit power is −2 dB and that thelower non-zero limit is −8 dB, and that power control commands toincrease the power above −2 dB during a dip in the SIR cannot be obeyed.In the region indicated at A′ the SIR of the second signal has dipped,the transmit power of the first signal has peaked as indicated at C′,and so the power control commands cannot be obeyed, and so the SIR ofthe first signal has dipped outside of the controlled range ±1 dB.

Referring to FIG. 4, there is illustrated the benefit of increasing therate at which channel quality reports are transmitted when the primarystation 100 fails to obey transmit power control commands. The abscissais the percentage of time for which the first signal is transmitted atmaximum power. The primary station 100 estimates the SIR of the firstsignal from the current transmit power of the first signal: If the powercontrol commands were being obeyed, the SIR of the first signal would beexpected to be proportional to the reciprocal of the transmitted power.The primary station 100 then uses channel quality reports received atregular intervals from the secondary station 200 to calibrate theestimated SIR in order to select suitable modulation and coding schemesfor the first signal or another signal. Each modulation and codingscheme is suitable for a small range of SIR values. If the error betweenthe estimated SIR and the actual SIR increases, the selected modulationand coding schemes become less suitable and the quality of service, forexample in terms of throughput or delay, of that signal deteriorates.The error arises because, for example, a relative transmit power of −2dB is expected (without any power constraint) to be required inconditions of a certain SIR, whereas the actual SIR is sometimes muchworse leading to a demand for a higher, unattainable transmit power. Theordinate is the RMS error between the actual SIR of the first signal andthe SIR for which the selected modulation and coding scheme is suitable.The RMS error should be minimised for best performance; ideally theactual SIR is equal to the SIR for which the selected modulation andcoding scheme is suitable. In FIG. 4 the RMS error is plotted for tworates of transmitting channel quality reports, 12.5 Hz and 500 Hz, foreach of two secondary station speeds, 3 km/h and 10 km/h. When a lowreporting rate is used, the estimated SIR is not able to be calibratedvery often, with the result that as the proportion of time spent atmaximum power increases the selected modulation and coding schemesbecome less suitable for the actual SIR. Hence when a low reporting rateis used, the RMS error between the actual SIR and the SIR for which theselected modulation and coding scheme is suitable increases. If a higherreporting rate is used the estimated SIR can be calibrated morefrequently by the channel quality reports so relatively suitablemodulation and coding schemes may be selected even when the proportionof time spent at the maximum available transmit power level of theprimary station 100 is high. Hence the RMS error is reduced byincreasing the reporting rate. It can be seen that the reduction in RMSerror is particularly evident at low speeds, where it is possible to usea sufficiently fast reporting rate relative to the rate of change ofchannel conditions to keep the estimated SIR correctly calibrated at alltime. Typically, the reporting rate might be increased when the primarystation 100 spends more than 15% of the time at its maximum availabletransmit power.

The term primary station in the present specification may denote a fixedstation in a mobile communication network and the term secondary stationmay denote a mobile station. Equally, the term secondary station in thepresent specification may denote a fixed station in a mobilecommunication network and the term primary station may denote a mobilestation.

The elements of a primary station relevant to the present invention neednot be co-located, but may be distributed within a communicationnetwork. Correspondingly, the elements of a secondary station relevantto the present invention need not be co-located, but may be distributedwithin a communication network.

In the present specification and claims the word “a” or “an” precedingan element does not exclude the presence of a plurality of suchelements. Further, the word “comprising” does not exclude the presenceof other elements or steps than those listed.

The inclusion of reference signs in parentheses in the claims isintended to aid understanding and is not intended to be limiting.

From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in the art of communicationswhich may be used instead of or in addition to features alreadydescribed herein.

1. A secondary station (200) for use in a communication systemcomprising a primary station (100) and a secondary station (200), thesecondary station (200) comprising: receiver means (240) for receiving afirst signal transmitted by the primary station (100); measurement means(250) for measuring a first characteristic of the received first signal;processing means (260) for deriving power control commands from themeasured first characteristic, transmitter means (210) for transmittingcontrol information comprising at least the power control commands tothe primary station (100); further comprising control means (270)responsive to the primary station (100) failing to adjust its transmitpower in compliance with the transmitted power control commands formodifying a parameter of the control information transmitted to theprimary station (100).
 2. A secondary station (200) as claimed in claim1, wherein the control means (270) is adapted to modify the parameter ofthe control information transmitted to the primary station (100) bygenerating an indication of a power step size for transmission to theprimary station (100) to use when adjusting its transmit power.
 3. Asecondary station (200) as claimed in claim 1, wherein the receivermeans (240) is adapted to receive a second signal transmitted by theprimary station (100); wherein the measurement means (250) is adapted tomeasure a second characteristic of the received second signal; whereinthe processing means (260) is adapted to derive channel quality reportsfrom the measured second characteristic, wherein the transmitter means(210) is adapted to transmit the channel quality reports to the primarystation (100) at a predetermined rate; and wherein the control means(270) is adapted to modify the parameter of the control informationtransmitted to the primary station (100) by causing the channel qualityreports to be transmitted at a rate higher than the predetermined rate.4. A secondary station (200) as claimed in claim 1, wherein the receivermeans (210) is adapted to receive a second signal transmitted by theprimary station (100); wherein the measurement means (250) is adapted tomeasure a second characteristic of the received second signal; whereinthe processing means (260) is adapted to derive channel quality reportsfrom the measured second characteristic, whereby each of the channelquality reports is derived by averaging a function of the measuredsecond characteristic over a predetermined time period, wherein thetransmitter means (210) is adapted to transmit the channel qualityreports to the primary station (100); and wherein the control means(270) is adapted to modify the parameter of the control informationtransmitted to the primary station (100) by causing the averaging to beperformed over a time period shorter than the predetermined time period.5. A secondary station (200) as claimed in claim 1, wherein the controlmeans (270) is adapted to detect failure of the primary station (100) toadjust its transmit power in compliance with the transmitted powercontrol commands.
 6. A secondary station (200) as claimed in claim 5,wherein the control means (270) is adapted to detect failure of theprimary station (100) to adjust its transmit power in compliance withthe transmitted power control commands by determining thesignal-to-interference ratio (SIR) of the received first signal and bydetecting when a function of the SIR fulfils a predetermined criterion.7. A secondary station (200) as claimed in claim 1, wherein the controlmeans (270) is adapted to receive an indication of the failure of theprimary station (100) to adjust its transmit power in compliance withthe transmitted power control commands.
 8. A secondary station (200) asclaimed in claim 1, wherein the control means (270) is adapted to storean indication of one or more predetermined times at which the primarystation (100) may fail to adjust its transmit power in compliance withthe transmitted power control commands and wherein the control means(270) is responsive to the occurrence of the one or more predeterminedtimes for modifying the parameter of the control information transmittedto the primary station (100).
 9. A communication system (50) comprisinga secondary station (200) as claimed in claim 1 and a primary station(100), the primary station (100) comprising: transmitter means (110) fortransmitting a first signal; receiver means (140) for receiving powercontrol commands; control means (150) for adjusting the transmit powerof the first signal in compliance with the received power controlcommands provided that the adjustment is within the contemporaneouscapability of the primary station (100); and wherein the primary stationcontrol means (150) is adapted to adapt a characteristic of atransmission in response to receiving the modified control information.10. A communication system (50) as claimed in claim 10, wherein theprimary station control means (150) is adapted to generate an indicationfor transmission in response to failing to adjust the transmit power ofthe first signal in compliance with the received power control commands.11. A primary station (100) for use in a communication system (50)comprising a primary station (100) and a secondary station (200), theprimary station (100) comprising: transmitter means (110) fortransmitting a first signal; receiver means (140) for receiving powercontrol commands; control means (150) for adjusting the transmit powerof the first signal in compliance with the received power controlcommands provided that the adjustment is within the contemporaneouscapability of the primary station (100); wherein the control means (150)is adapted to generate an indication for transmission in response tofailing to adjust the transmit power of the first signal in compliancewith the received power control commands.
 12. A method of operating acommunication system (50) comprising a primary station (100) and asecondary station (200), the method comprising: transmitting a firstsignal from the primary station (100); at the secondary station (200),receiving the first signal; while receiving the first signal, measuringa first characteristic of the received first signal, deriving powercontrol commands from the measured first characteristic, andtransmitting control information comprising at least the power controlcommands; at the primary station (100), receiving the power controlcommands; adjusting the transmit power of the first signal in compliancewith the received power control commands provided that the adjustment iswithin the contemporaneous capability of the primary station (100);further comprising, at the secondary station (200), in response to theprimary station (100) failing to adjusting the transmit power incompliance with the received power control commands, modifying aparameter of the control information transmitted to the primary station(100), and at the primary station (100), in response to receiving themodified control information, adapting a characteristic of atransmission.
 13. A method as claimed in claim 12, wherein modifying theparameter of the control information transmitted to the primary station(100) comprises transmitting an indication of a power step size for useby the primary station (100) when adjusting its transmit power.
 14. Amethod as claimed in claim 12, further comprising: transmitting a secondsignal from the primary station (100); at the secondary station (200),receiving the second signal; while receiving the second signal,measuring a second characteristic of the received second signal,deriving channel quality reports from the measured secondcharacteristic, and transmitting the channel quality reports at apredetermined rate; at the primary station (100), receiving the channelquality reports; determining at least one parameter of the first signalor another signal in response to the channel quality reports; whereinmodifying the parameter of the control information transmitted to theprimary station (100) comprises transmitting the channel quality reportsat a rate higher than the predetermined rate.
 15. A method as claimed inclaim 12, further comprising: transmitting a second signal from theprimary station (100); at the secondary station (200), receiving thesecond signal; while receiving the second signal, measuring a secondcharacteristic of the received second signal, deriving channel qualityreports from the measured second characteristic whereby each of thechannel quality reports is derived by averaging a function of themeasured second characteristic over a predetermined time period, andtransmitting the channel quality reports; at the primary station (100),receiving the channel quality reports; determining at least oneparameter of the first signal or another signal in response to thechannel quality reports; wherein modifying the parameter of the controlinformation transmitted to the primary station (100) comprises averagingthe function of the measured characteristic over a time period shorterthan the predetermined time period.
 16. A method as claimed in claim 12,comprising detecting at the secondary station (200) the failure of theprimary station (100) to adjust the transmit power in compliance withthe received power control commands.
 17. A method as claimed in claim16, wherein detecting the failure comprises determining thesignal-to-interference ratio (SIR) of the received first signal anddetecting when a function of the SIR fulfils a predetermined criterion.18. A method as claimed in claim 12, comprising detecting at the primarystation (100) the failure of the primary station (100) to adjust thetransmit power in compliance with the received power control commandsand, in response to detecting the failure, transmitting to the secondarystation (200) an indication of the failure.
 19. A method as claimed inclaim 12, wherein times at which the primary station (100) fails toadjust the transmit power in compliance with the received power controlcommands are predetermined.